The term "distribution transformer," often abbreviated as "distribution," refers to a static electrical apparatus that transfers alternating current (AC) energy via electromagnetic induction, altering voltage and current levels. In certain regions, power transformers operating at voltages below 35 kV (most commonly 10 kV or lower) are termed "distribution transformers" or simply "distributions." These transformers are typically installed either on poles or open floors within substations. Details regarding their installation methods, precautions, supply distribution approaches, capacity selection, operational maintenance, and more are thoroughly discussed.
A comprehensive explanation of distribution transformers and their capacity selection is provided. Firstly, the 10 kV high-voltage power grid uses a three-phase, three-wire neutral-point ungrounded system. However, most user transformers adopt a D/yn11 connection mode with a directly grounded neutral point, enabling a three-phase four-wire power supply system.
When selecting the capacity of a distribution transformer, issues may arise from either oversized transformers running underloaded or equipment overheating and even burning out due to overload or overcurrent operations. Improper capacity selection can impact the reliability and economic efficiency of the power system. Transformer capacity is generally chosen based on load statistics, often based on anticipated maximum loads, leading to capacities that are frequently too large, negatively affecting the power system's operation. Selecting for economic operation involves deriving the maximum economic load rate using the transformer's copper and iron losses, but this approach is challenging since real-world loads vary randomly, making it difficult to achieve optimal efficiency.
Currently, replacing high-energy transformers with new low-loss models in power distribution systems has significantly reduced single-iron losses by about 40%. Given the large number of distribution transformers and fluctuating loads, the economic benefits are substantial.
To ensure the transformer operates efficiently without compromising its lifespan, selecting its capacity based on maximizing utilization while maintaining normal service life should be prioritized. A recommended method is to choose the distribution transformer capacity following the maximum expected load (Smax) and a typical daily load curve, aligning with the International Electrotechnical Commission (IEC) standard from 1972, which includes an oil-immersed transformer load guide. This method considers the transformer’s normal overload capacity, fully utilizing its potential without shortening its life. This approach reduces investment, improves distribution network conditions, and offers significant economic advantages.
Through a computer program based on this method, a transformer capacity selection table corresponding to six typical daily load curves has been developed, along with reference types for negative curves. These include:
I: Irrigation, wheat fields;
II: Village industrial payments, lighting, courtyard use;
III: Payment for labor, lighting, irrigation, and courtyard use;
IV: Land and county industrial use;
V: Village integrated loads with industrial components;
VI: Urban industrial composite load.
The scheduling method is as follows:
1. Identify the load type and select a typical daily load curve.
2. Determine the equivalent air temperature (θδ); the IEC standard's equivalent air temperature is not the average environmental temperature but reflects insulation degradation under varying air temperatures. For simplicity, 22°C is suggested for the Jiangnan region, 20°C for Jiangbei, and 16°C and 18°C for the northwest and northeast regions.
3. Using the expected maximum load value (in kVA), consult the table to determine the selected transformer's nominal capacity (Sn).
For example: Load curve I, with an annual equivalent air temperature of 9°C and a maximum load of 1000 kVA, suggests selecting an 800 kVA distribution transformer.
4. Based on environmental conditions and the nature of the load, determine the normal overload capacity of the working transformer.
For instance, Class VI load curve with an annual equivalent air temperature of 22°C, the working transformer has a front angle capacity of 315 kVA and a maximum load band of 340 kVA.
It is important to note that transformer capacity should be selected following the aforementioned method, with the maximum load continuous operation time ensuring safety. Exceeding this time poses a risk of transformer burnout. The allowable time can be calculated using the natural circulation oil-immersed transformer winding's maximum temperature formula. For convenience, tables have calculated the maximum load limit running time (τmaxe). The condition assumes an ambient temperature of 35°C, with the winding hot spot temperature not exceeding 140°C. For example, Schedule IV with an annual equivalent air temperature of 20°C allows a maximum load limit running time of 17 hours when the ratio of maximum load to rated capacity is 3dl. Below a ratio of 1.17, the maximum load is extremely limited, posing no risk of overheating.
In conclusion, proper transformer capacity selection is critical for ensuring efficient and safe operation within power systems, balancing economic considerations with long-term sustainability.
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